But it turns out that Twitter still has a mobile gap, just like everyone else: When people use Twitter on their phones, they generate less money for the company.

Twitter doesn’t spell out the size of its mobile gap, but it does talk about it in its S-1 document, filed yesterday. The big problem: Two of Twitter’s primary ad products — “Promoted Accounts” and “Promoted Trends” — don’t work very well on mobile phones because it’s hard for people to see them.

So that means that Twitter has “generated higher advertising revenue per timeline view on our desktop applications than on our mobile applications,” the company said.

And because 75 percent of Twitter users get to the service via a mobile device (at least some of the time), and because that number is likely to increase, the problem could become more acute: “To the extent that user engagement on mobile applications continues to increase faster than user engagement on our desktop applications, advertising revenue per timeline view may be adversely impacted even if total advertising revenue continues to increase.” (Correction: I previously reported that 65 percent of Twitter usage happens on mobile, which is incorrect. Twitter says it generates 65 percent of its ad revenue from mobile.)

Twitter may be able to address some of this via new ad products that work better on phones. And changes to Twitter itself may help.

As my colleague Mike Isaac has reported, an upcoming makeover for Twitter’s iPhone software will put more focus on images and video, which could have obvious ad implications. If Twitter can figure out how to make its mobile “discover” section more usable, that would also help, since that’s where the company currently stashes Promoted Trend and Account ads.

But some parts of the mobile problem will be beyond Twitter’s control. The majority of the company’s user base — 77 percent in Q2 2013 — is outside the U.S. And in some of those markets, users can’t get access to Twitter goodies like video, because they’re on slow Web connections or crude phones or both.

That’s presumably one of the reasons Twitter’s international users only account for 25 percent of the company’s revenue, and why “advertising per timeline view” — Twitter’s new version of an ad metric — generates $2.17 in the U.S. and 30 cents everywhere else in the world.

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